1. Field of the Invention
The present invention generally relates to compositions and methods used for hydrocarbon exploitation such as in the drilling of and production from wells, especially oil and gas wells. More particularly, the invention relates to such compositions and methods which alter the physical or chemical properties of a polymeric component of an oil field fluid or residue, such as decomposing a polymeric viscosifier or fluid loss control agent contained in such fluid or residue in response to a defined chemical or physical signal.
2. Description of Related Art
The selection of materials for well construction is essential to the successful completion of an oil or gas well. Among the most important is the selection of a drilling fluid. A drilling fluid having the desired properties is passed down through the drill pipe, out a nozzle at the drill bit, and returned to the surface through an annular portion of the well bore. The drilling fluid primarily functions to remove cuttings from the bore hole; lubricate, cool and clean the drill bit; reduce friction between the drilling string and the sides of the bore hole; maintain stability of the bore hole; prevent the inflow of fluids from permeable rock formations; and provide information on downhole conditions. The composition of a drilling fluid is carefully selected to optimize production within the vast diversity of geological formations and environmental conditions encountered in oil and gas recovery. At the same time, the fluid should not present a risk to personnel, drilling equipment, or the environment.
Drilling fluids may be water, oil, synthetic, or gas based. The composition is typically tailor-made to specific drilling conditions, varying in size and distribution of suspended particles, density, temperature, pH, pressure, salt concentration, alkalinity, electrical conductivity, lubricity, and corrosivity, all of which may be influenced by the surrounding geological formations. Further explanation of the properties of fluids useful in the recovery of oil and gas may be obtained from a review of the publication, H. C. H. DARLEY and GEORGE R. GRAY, COMPOSITION AND PROPERTIES OF DRILLING AND COMPLETION FLUIDS 1-37 (5th ed. 1988); and CHILINGARIAN, ET AL., DRILLING AND DRILLING FLUIDS, DEVELOPMENTS IN PETROLEUM SCIENCE 11 (1981).
Water-based drilling fluids, or muds, may consist of polymers, biopolymers, clays and organic colloids added to an aqueous based fluid to obtain the required viscous and filtration properties. Heavy minerals, such as barite or calcium carbonate, may be added to increase density. Solids from the formation are incorporated into the mud and often become dispersed in the mud as a consequence of drilling. Further, drilling muds may contain one or more natural and/or synthetic polymeric additives, including polymeric additives that increase the rheological properties (e.g., plastic viscosity, yield point value, gel strength) of the drilling mud, and polymeric thinners and flocculents.
Polymeric additives included in the drilling fluid may act as fluid loss control agents. Fluid loss control agents, such as starch, prevent the loss of fluid to the surrounding formation by reducing the permeability of filter cakes formed on the newly exposed rock surface. In addition, polymeric additives are employed to impart sufficient carrying capacity and thixotropy to the mud to enable the mud to transport the cuttings up to the surface and to prevent the cuttings from settling out of the mud when circulation is interrupted.
Most of the polymeric additives employed in drilling mud are resistant to biodegration, extending the utility of the additives for the useful life of the mud. Specific examples of biodegradation resistant polymeric additives employed include biopolymers, such as xanthans (xanthan gum) and scleroglucan; various acrylic based polymers, such as polyacrylamides and other acrylamide based polymers; and cellulose derivatives, such as dialkylcarboxymethylcellulose, hydroxyethylcellulose and the sodium salt of carboxy-methylcellulose, chemically modified starches, guar gum, phosphomannans, scleroglucans, glucans, and dextrane. See U.S. Pat. No. 5,165,477, which is incorporated herein by reference.
Most drilling fluids are designed to form a thin, low-permeability filter cake to seal permeable formations penetrated by the bit. This is essential to prevent both the loss of fluids to the formation and the influx of fluids that may be present in the formation. Filter cakes often comprise bridging particles, cuttings created by the drilling process, polymeric additives, and precipitates.
For a filter cake to form, it is important that the mud contain bridging particles, particles of a size selected to seal the pore openings in the formation. While finer particles may be carried deeper into a formation, bridging particles are trapped in the surface pores, and form the foundation for the filter cake. The bridged zone in the surface pores begins to trap successively smaller particles, and fluids interchange until an essentially impenetrable barrier is formed.
The formation of a filter cake seal is fostered by an imbalance of pressure of the mud column over the pressure exerted by fluids within the formation. It is recommended that drilling fluid pressure exceed the pressure exerted by fluids in the pores of the formation by about 200 psi. Pore pressure depends on the depth of the formation, the density of the pore fluids, and geological conditions. Similarly, the outward pressure exerted by the drilling fluid is a function of the density of the drilling fluid and the depth of the formation in question.
Since the outward pressure of the mud column is usually greater than the pressure exerted by the pore formation, it is also a primary function of the filter cake to prevent drilling fluid from continuously permeating into formations surrounding the well bore. The permeability of the filter cake is dependent upon particle distribution and size, in addition to electrochemical conditions of the mud. The composition of the drilling fluid can be adjusted to increase or decrease permeability, for example, by adding soluble salts, or increasing the number of particles in the colloidal size range. Fluid from the mud which permeates the barrier is known as filtrate. The probability of successful completion of a well may depend, in large part, upon the filtration properties of the mud being matched to the geological formations, and the composition of the filtrate. For further explanation of the properties and formation of filter cakes, see H. C. H. Darley and George R. Gray, COMPOSITION AND PROPERTIES OF DRILLING AND COMPLETION FLUIDS, (5th ed., 1988).
Although filter cake formation is essential to drilling operations, the filter cake can be a significant impediment to the production of hydrocarbon or other fluids from the well. Damage to producing formations can occur by directly plugging the surface of the rock, M. J. Economides, et al., PETROLEUM WELL CONSTRUCTION, John Wiley and Sons, N.Y., 1988, p.121, or indirectly by plugging the hardware placed in the well. Ladva, H. K. J., et al., xe2x80x9cMechanisms of Sand Control Screen Plugging From Drill-In Fluids and its Cleanup Using Acid, Oxidizers and Enzyme Breakers,xe2x80x9d SPE 39439 (Feb. 18, 1998). Removal of the blockage presented by the filter cake may be essential to the commercial viability of the well. Many methods are used to remove filter cake damage, including concentrated acids, strong oxidizers, chelating agents and enzymes. Because enzymes are highly specific, they do not react or degrade the materials commonly found within a subterranean formation or used in well bore operations, such as limestone, iron, resin coated proppants, tubings and the like. This makes enzymes an excellent candidate to destroy the filter cake without harming the completion hardware or personnel.
As disclosed by U.S. Pat. No. 5,247,995 (xe2x80x9cthe ""995 patentxe2x80x9d), incorporated herein by reference, the permeability of a formation may be assessed in a laboratory. One procedure of assessing the permeability measures the flow of a fluid through a damaged formation sample at a given rate and pressure. As reported, a completely broken filter cake regains greater than about 95% of the initial permeability of a formation sample using a damage permeability test, while a plugged formation has about 30% of the initial permeability, depending on the fluid, core and conditions. A second procedure assesses the retained conductivity of the formation. As reported, a plugged formation has retained conductivity of less than 10%, depending on the conditions.
Therefore, removal of the filter cake is necessary to increase flow of production fluids from the formation. Since filter cake is compacted and often adheres strongly to the formation, it may not be readily or completely flushed out of the formation by fluid action alone. Removal of the filter cake often requires some additional treatment. Common oxidants, for example, persulfates, may be used to remove filter cake. As the ""995 patent disclosed, however, oxidants are ineffective at low temperature ranges, from ambient temperature to 130xc2x0 F. As reported, in this temperature range the oxidants are stable and do not readily undergo homolytic cleavage to initiate the degradation of the filter cake. Cleavage is typically achieved at lower temperatures only by using high concentrations of oxidizers. High oxidizer concentrations are frequently poorly soluble under the treatment conditions.
Reactions involving common oxidants are also often difficult to control. Oxidants tend to react with many things other than their intended target. For example, oxidants can react with iron found in the formation, producing iron oxides that precipitate and damage the formation, decreasing permeability. Oxidants can also react non-specifically with other materials used in the oil industry, for example, tubings, linings and resin coated proppants.
Further, to completely remove the filter cake after treating with oxidants, additional treatment may be required. An extra acid hydrolysis step may be necessary to remove any residue. Treatment with an acid, for example, hydrochloric acid, augments the removal of excess residue. Acid treatments, however, corrode steel and equipment used in the operation. Acid treatments may also be incompatible with the formation and/or its fluids.
Residues, such as filter cakes, can also present difficulties during drilling operations. For example, in permeable formations, filtration properties must be controlled to prevent thick filter cakes from excessively reducing the gauge of the borehole. Further, poor filter cakes may cause the drill pipe to become stuck, known as xe2x80x9cdifferential sticking.xe2x80x9d Helmick and Longley, xe2x80x9cPressure-Differential Sticking of Drill Pipe and How it Can Be Avoided or Relieved,xe2x80x9d API Drill. Prod. Prac. (1957). pp.55-60; Outmans, H. D., xe2x80x9cMechanics of Differential-Pressure Sticking of Drill Collars,xe2x80x9d Trans. AIME, Vol. 213 (1958). pp.265-274. This occurs when part of the drill string bears against the side of the hole while drilling, and erodes away part of the filter cake. When rotation of the pipe is stopped, the part of the pipe in contact with the cake is isolated from the pressure of the mud column, and is subject only to the pore pressure of the filter cake. The differential pressure thus created causes drag which can be sufficient to prevent the pipe from being moved. Sometimes, the pipe can be freed by spotting oil around the stuck section, but if this procedure fails, more expensive and time consuming methods are entailed (H. C. H. DARLEY and GEORGE R. GRAY, COMPOSITION AND PROPERTIES OF DRILLING AND COMPLETION FLUIDS 405-11 (5th ed. 1988)).
In addition, drilling fluid residues remaining in the well tend to interfere with other phases of drilling and completion operations such as cementing the casing to the wall of the bore. Filter cake and residual mud can prevent casing cement from properly bonding to the wall of the bore. The trajectory of a well bore may be tortuous, and the wall of the bore often has various ledges and cavities therein which contain thixotropic drilling mud. The drilling mud in contact with the bore wall is quiescent while the casing is lowered into the bore and tends to gel. When circulation is resumed, the fluid pumped through the casing and up through the annulus between the casing and the bore wall makes paths or channels or even bypasses the xe2x80x9cgelledxe2x80x9d mud contained by the ledges and cavities.
Thus, cement pumped through the casing and up through the annulus to cement the casing to the bore wall flows through the paths or channels in the mud leaving large pockets of mud between the casing and the bore wall. These pockets can ultimately result in fluid communication with formation zones that the cement is supposed to isolate.
In an attempt to solve the above-noted problem, special fluids are often circulated through the annulus between the casing and the wall of the bore before the casing is cemented to remove mud remaining therein. Unfortunately, this procedure, often referred to as a xe2x80x9cspacerxe2x80x9d flush, is inadequate in many applications. Conventional flushing fluids are not always capable of sufficiently decreasing the gel strength, viscosity and other rheological properties of the mud caused by polymeric additives therein. As a result, the mud cannot be flushed out of the well. Instead, expensive squeeze cementing operations are carried out to fill in the gaps in the cement caused by the mud. For example, see U.S. Pat. No. 5,165,477, incorporated herein by reference.
Enzymes arc a class of proteins that are responsible for catalyzing almost every chemical reaction that occurs in living organisms. They are characterized by two remarkable qualities: (1) to act as catalysts, often increasing the rate of a chemical reaction by as much as 106-1012 times that of an uncatalyzed reaction; and (2) their high degree of specificity, the ability to act selectively on one substance or a small number of chemically similar substances. As a catalyst, enzyme structure remains unaltered as a result of reaction with the substrate, thus, the enzyme may initiate another reaction, and so on. However, as nature""s catalysts, enzymes are usually only active within the range of conditions, particularly pH, temperature, and aqueous solvents, found within the cells from which they are isolated. While the range of environmental conditions in which living organisms exist is quite broad, this presents a major distinction between enzymes and other chemical catalysts, such as charcoal and platinum, which often require much higher temperatures and more extreme pH conditions than most enzymes. For a more detailed discussion of the properties of enzymes, see LODISH, ET AL., MOLECULAR CELL BIOLOGY, 75-86 (3d ed. 1995).
It has been reported in the literature that enzymes can be used to degrade drilling fluid residues. For example, Hanssen, et al., xe2x80x9cNew Enzyme Process for Downhole Cleanup of Reservoir Drilling Filter cakexe2x80x9d SPE 50709 (1999) describes experimental work towards the use of enzymes for downhole cleanup of filter cakes produced by water-based drilling fluids. These experiments focused on filter cakes containing modified starch and xanthan, applying thermostable xcex1-amylases, and polyanionic cellulose (PAC)-based fluids using cellulase enzymes. As reported, these enzymes are shown to be highly effective in degrading starch/xanthan and PAC/xanthan water-based drilling fluids and their filter cakes in the laboratory.
Hanssen, et al., disclosed the properties of several enzymes and filter cake components as follows:
All starches are mixtures of amylose, a linear polysaccharide, and the related but branched amylopectin, in a ratio dependent on its natural source (corn, potatoes, and other crops). Molecular weight also varies with the source, but is typically very high: 105-109 corresponding to approx. 500-5000 monomer units. Chemically modified starches may have hydroxyethyl or hydroxypropyl side-chain substituents on an unchanged backbone. Modified and crosslinked starches may be as large as 30xcexc in size.
An xcex1-amylase enzyme is reported to hydrolyze the xcex1-1,4 glycosidic bonds characteristic of the starch backbone to water-soluble oligosaccharides of 2 to 10 sugar units. It is indicated that the reaction occurs by attachment of the active site in the enzyme to an xcex1-1,4 bond in the polymer molecule where hydrolysis can occur, forming an enzyme-substrate complex, followed by xe2x80x9cclippingxe2x80x9d of the bond. This reaction continues on and on again, causing the degradation of the polymer chain. These enzymes typically have molecular weights on the order of 25-75,000 and diameters of 5-10 nm. Hence, amylases are smaller than the polysaccharides they destroy, but have a very different shape.
Cellulase enzymes are similarly reported as specific for the bonds in cellulose polymers. Here the xcex2-(1,4) bonds characteristic of this polysaccharide are broken down. Carboxymethyl celluloses (CMC""s) and polyanionic celluloses (PAC""s) in general, with hydrophilic side chains, were also degraded by the celluloses reported in the Hanssen, et al., study.
In addition to their conclusions as to the potential of enzymes in oil production, Hanssen, et al., disclosed two experimental methods which allow for rapid, repeatable and consistent selection and development of enzyme products for application in the field, including (1) a visual filter cake degradation test for screening of treatment fluid, and (2) filtration tests for quantitative evaluation of enzyme activity.
Others have also described the useful properties of enzymes. U.S. Pat. No. 5,126,051, and U.S. Pat. No. 5,165,477, both of which are incorporated herein by reference, disclose the use of enzymes for (1) cleaning up a well site drilling mud pit containing drilling mud comprising polymeric organic viscosifiers; and (2) removing used drilling mud comprising a polymeric organic viscosifier from a wellbore. In the downhole application of this invention, a fluid comprising one or more enzymes capable of rapidly degrading the polymeric organic component of the drilling fluid is injected into the well. The enzymes degrade the organic polymeric viscosifier, allowing the drilling fluid residues to disperse within a wash fluid, which can then be recovered from the well. As disclosed, the enzymes contained within the fluid wash must rapidly decompose the drilling mud in contact with the wellbore before they are rendered inactive by harsh downhole conditions. As reported, laboratory tests conducted using five different enzymes illustrated that enzymes can be effectively used at low concentrations to rapidly degrade polymeric organic viscosifiers of the type used in drilling muds.
Further, U.S. Pat. No. 5,247,995 (xe2x80x9cthe ""995 patentxe2x80x9d), incorporated herein by reference, discloses a method of degrading damaging polysaccharide-containing filter cakes, produced from fracturing fluids, and other damaging fluids using enzymes specific to those polysaccharides. The method consists of pumping an enzyme treatment to a desired location within the well bore to coat the filter cake, degrading the polysaccharide containing filter cake, and removing the degraded filter cake, thus increasing the permeability of the formation.
Specifically, the ""995 patent describes suitable hydratable polysaccharides such as the galactomannan gums, guars, derivatized guars, cellulose and cellulose derivatives. Specific examples disclosed are guar gum, guar gum derivatives, locust bean gum, caraya gum, xanthan gum, cellulose, and cellulose derivatives. Further, the invention of the ""995 patent describes various other suitable polysaccharides used in the oil industry, such as starch and starch derivatives, which thicken fluids and control fluid loss.
The method of the ""995 patent for treating guar-containing filter cakes comprises using enzymes that are hydrolases. As reported, the enzyme hydrolases are stable in the pH range of about 2,0 to 11.0 and remain active at both acid and alkaline pH ranges of about 2.0 to 10.0. These same enzymes were reported as active at low to moderate temperatures of about 50xc2x0 F. to about 195xc2x0 F. As disclosed, for the preferred method of the ""995 patent, the pH range is 3 to 7 at a temperature range of about 80xc2x0 F. to 195xc2x0 F. At temperatures of above about 125xc2x0 F., the preferable pH ranges from about 3 to 5.
As disclosed, the enzymes are specific to attack the mannosidic and galactomannosidic linkages in the guar residue, breaking the molecules into monosaccharide and disaccharide fragments. Under some conditions, these enzymes hydrolyze the residue completely into monosaccharide fragments. The preferred enzymes for the guar-containing filter cake are galactomannan hydrolases collectively called galactomannanase and they specifically hydrolyze the (1,6)-xcex1-D-galactomannosidic and the (1,4)-xcex2-D-mannosidic linkages between the monosaccharide units in the guar-containing filter cake respectively.
The method of the ""995 patent also consists of removing cellulose-containing filter cakes using hydrolase enzymes which differ from the enzymes for the guar-containing filter cake. As reported, these enzymes are active in the pH range of about 1.0 to 8.0. The preferred pH range is about 3.0 to 5.0. These same enzymes are active at low to moderate temperatures of about 50xc2x0 F. to 140xc2x0 F. Most preferably for the method of the invention, the pH is about 3.5 to 4.0.
As disclosed by the ""995 patent, with a cellulose or derivatized cellulose containing filter cake, the specific enzymes attack the glucosidic linkages of the cellulose backbone, breaking the backbone into fragments. Insoluble cellulose is composed of repeating units of D-glucose joined by (1,4)-xcex2-glucosidic linkages. The fragments are broken down into soluble D-glucose monosaccharides. The preferred enzymes are any enzymes or combination of enzymes that attack the glucosidic linkages of the cellulose polymer backbone and degrade the polymer into mostly monosaccharide units, such as cellulase, nonspecific hemicelluases, glucosidase, endoxylanase, exo-xylanase and the like. The two preferred enzymes are commonly called exo and endo xylanases. The preferred enzymes for this cellulose based system specifically hydrolyze the exo(1,4)-xcex2-D-glucosidic and the endo(1,4)-xcex2-D-glucosidic linkages between the monosaccharide units in the cellulose backbone and the (1,4)-xcex2-D-glucosidic linkage of any cellobiose fragments.
Further, the method of the ""995 patent for removing starch derived filter cake consists of using enzymes that are specific for the linkages found within the starch molecule. These enzymes are active at the pH range of between about 2.0 to 10.0 for the temperature range of about 50xc2x0 F. to 230xc2x0 F.
As described, starch, like cellulose, is a polysaccharide formed of repeating units of D-glucose. However, the glucose molecules are joined in an (1,4)-xcex1-glucosidic linkage rather than the (1,4)-xcex2-glucosidic linkage found in cellulose. Starch contains a mixture of two polymers, amylose and amylopectin. Amylose consists of a linear chain of D-glucose molecules bound in xcex1-D-(1-4) linkages. Amylopectin, the major component of the starch polysaccharide, is a highly branched D-glucan with a backbone of D-glucose xcex1-D-(1-4) linkages and D-glucose side chains connected by xcex1-D-(1-6) linkages. To reduce the viscosity of starch residue, such as filter cake, the preferred enzymes digest the starch molecules until no starch is present as determined by iodine testing. The enzymes reduce the starch into smaller units, most likely oligosaccharide units and dextrin. This degradation sufficiently decreases the size of the starch polymer so as to make it soluble, removing it as component in the filter cake. The smaller polysaccharides do not damage the formation and often terminally degrade at higher temperatures. These enzymes or combination of enzymes are selected from the endo-amylases, exo-amylases, isoamylases, glucosidases, xcex1-glucosidases, glucan (1,4)-xcex1-glucosidase, glucan (1,6)-(xcex1-glucosidase, oligo-(1,6)-glucosidase, xcex1-glucosidase, xcex1-dextrin endo-(1,6)-xcex1-glucosidase, amylo-(1,6)-glucosidase, glucan (1,4)-xcex1-maltotetrahydralase, glucan (1,6)-xcex1-isomaltosidase, glucan (1,4)-xcex1-maltohexaosidase, and the like.
As disclosed, the preferred enzymes are endo-amylases. The endo-amylases randomly attack the internal xcex1-glucosidic linkages. There is no preferable type of endo-amylase, as the specific endo-amylase selected varies on the conditions present in the formation, such as pH and temperature.
Further, as disclosed, the enzyme treatment for cellulose-containing polysaccharides can be adapted for other polysaccharides with the cellulose backbone and side chains. The treatment may require additional enzymes to break the side chain linkages before effective degradation of the backbone occurs. These enzymes are hydrolases specific to the linkages of the side chains.
One example disclosed in the ""995 patent of this type of polysaccharide is xanthan. Enzyme treatment specific for the xanthan polysaccharide reduces the static viscosity of the xanthan. As described, the enzyme treatment works at a pH range between about 2.0 and 10.0 at temperatures ranging from about 50xc2x0 F. to 150xc2x0 F.
As described in the ""995 patent, xanthan gums are cellulose-containing, heteropolysaccharides. Xanthans contain a cellulose backbone of (1,4)-xcex2-D-glucosidic linkages and trisaccharide side chains on alternate residues. The trisaccharide side chains may consist of glucuronic acid, pyruvated mannose, mannose, and/or acetylated mannose. The method of the ""995 patent uses hydrolases which can break down the (1,4)-xcex2-D-glucosidic linkages within the cellulose backbone. The cellulose backbone, however, can only be broken after treating the xanthan to degrade the trisacchanrde side chains with another enzyme such as a mannosidase. The treatment therefore requires at least two enzymes. The enzyme treatment uses the same enzymes described above for cellulose-containing filter cakes and mannosidase or mannan (1,2)-xcex2-D-mannosidase, although no particular enzymes or concentration of enzymes are currently preferred. The xanthan gum reduces to smaller polysaccharide molecules, probably the smallest is a tetrasaccharide. The degradation decreases the static viscosity of the xanthan polysaccharide for easy removal. The pH depends on the activity range of the selected enzymes nd the conditions found within the formation.
Further, U.S. Pat. No. 5,566,759, incorporated herein by reference, discloses a mechanism for degrading cellulose-containing fluids used during fracturing, workover and completion operations to produce an efficacious degradation of a cellulose-containing fluid at an alkaline pH range and higher temperatures than were disclosed in the ""995 patent, illustrating that systems can be designed for the use of enzymes which operate outside previously determined ranges of enzyme activity.
Methods of enzyme inactivation and encapsulation have been reported in the context of well stimulation and fracturing fluids.
Hydraulic fracturing is a conventional practice for producing one or more cracks or xe2x80x9cfracturesxe2x80x9d in a formation by applying sufficient pressure via a fracturing fluid to cause the mechanical breakdown of a formation. The fracturing process is meant to increase the permeability or conductivity of the formation, and ultimately, well productivity. Fracturing fluids are usually a highly viscous gel emulsion or foam, suspended in which is a proppant, such as sand or other particulate matter. The high-viscosity of the fluid is important, generating larger fracture volume and fracture width, and more efficiently transporting proppant material. The purpose of the proppant is to prevent the fracture from closing upon removal of pressure. Once the fracture has been established, it is desirable to remove the highly viscous fluid, allowing hydrocarbon production through the pores between the proppant in the newly formed fracture. To facilitate removal of the fluid, a xe2x80x9cbreaker,xe2x80x9d or viscosity-reducing agent, is employed. The typical breakers that are used in fracturing fluids are enzymes and oxidizers. Simply adding a breaker to the fluid, however, is problematic; results are often unreliable, and can lead to premature breaking of the fluid before the fracturing process is complete, resulting in a decrease in the number or length of fractures, and well productivity.
There have been a number of proposed methods for controlling the activity of breakers to alleviate the above problems. For example, U.S. Pat. No. 4,202,795, incorporated herein by reference, discloses a method in which a breaker is combined with a hydratable gelling agent, and a gel-degrading substance. The mixture is formed into pills or pellets, preferably having size and range of about 20 to about 40 mesh. (U.S. Sieve Series) After combining the pellets with an aqueous fluid into which the chemical is to be released, the gelling agent in the pellets hydrates and forms a protective gel around each of the pellets which prevents the release of the chemical into the aqueous fluid for the predetermined time period required for the protective gel to be removed by the gel-degrading substance in the pellets. The most serious problem associated with this system is that the breaker tends to be released over a significant period of time due to differences in the thickness of the protective coating and in variations of length of time and temperature exposure of the individual pellets. A large amount of hydratable gelling agent is typically required and the amount of hydratable gelling agent must be monitored closely.
U.S. Pat. No. 4,506,734, incorporated herein by reference, also provides a method for reducing the viscosity and the resulting residue of an aqueous or oil based fluid introduced into subterranean formation by introducing a viscosity-reducing chemical contained within hollow or porous, crushable and fragile beads along with a fluid, such as a hydraulic fracturing fluid, under pressure into the subterranean formation. When the fracturing fluid passes or leaks off into the formation, or the fluid is removed by back flowing, the resulting fractures in the subterranean formation close and crush the beads. The crushing of the beads then releases the viscosity-reducing chemical into the fluid. This process is dependent upon the closure pressure of the formation to obtain release of the breaker and is, thus, subject to varying results dependent upon the formation and its closure rate.
U.S. Pat. No. 4,741,401, incorporated herein by reference, discloses a method for breaking a fracturing fluid comprised of injecting into the subterranean formation a capsule comprising an enclosure member containing the breaker. The enclosure member is sufficiently permeable to at least one fluid existing in the subterranean environment or injected with the capsule such that the enclosure member is capable of rupturing upon sufficient exposure to the fluid, thereby releasing the breaker. The patent teaches that the breaker is released from the capsule by pressure generated within the enclosure member due solely to the fluid penetrating into the capsule whereby the increased pressure caused the capsule to rupture, i.e., destroys the integrity of the enclosure member, thus releasing the breaker. This method for release of the breaker would result in the release of substantially the total amount of breaker contained in the capsule at one particular point in time.
In another method to release a breaker, U.S. Pat. No. 4,770,796, incorporated herein by reference, teaches or suggests an acid fracturing fluid composition comprising a polymer, a crosslinking agent for said polymer, an aqueous acid and a breaker compound capable of coordinating with titanium or zirconium crosslinking agent. The breaker compound is encapsulated in a composition comprising a cellulosic material, a fatty acid, and, optionally, a wax.
Further, U.S. Pat. No. 4,919,209, incorporated herein by reference, discloses a proposed method for breaking a fracturing fluid. Specifically, the patent discloses a method for breaking a gelled oil fracturing fluid for treating a subterranean formation which comprises injecting into the formation a breaker capsule comprising an enclosure member enveloping a breaker. The enclosure member is sufficiently permeable to at least one fluid existing in the formation or in the gelled oil fracturing fluid injected with the breaker capsule, such that the enclosure member is capable of dissolving or eroding off upon sufficient exposure to the fluid, thereby releasing the breaker.
U.S. Pat. No. 5,102,558, incorporated herein by reference, discloses an encapsulated breaker chemical composition for use in a fracturing process. The capsule is described as a pinhole free coating of a neutralized sulfonated elastomeric polymer having a preferred thickness of about 2 to 80 microns deposited on the surface of a breaker chemical. The neutralized sulfonated polymer is not degraded by the breaker chemical, and is permeable to the breaker chemical at conditions of use.
U.S. Pat. No. 5,102,559, incorporated herein by reference, improves upon the neutralized sulfonated polymer capsule of U.S. Pat. No. 5,102,558 by first coating the breaker with a water soluble sealing layer, such as urea, such that the breaker is protected from aging and is prevented from degrading the polymer coating. Further, the seal shields the chemical from premature release by creating a barrier to water soluble fluid components.
Similarly, U.S. Pat. No. 5,110,486, incorporated herein by reference, describes an encapsulated breaker composition comprising a breaker chemical encapsulated by a pinhole free coating of an ionically and covalently crosslinked neutralized sulfonated elastomeric polymer. Again, the polymer is permeable to the breaker, which is non-reactive to the polymer.
U.S. Pat. No. 5,164,099, incorporated herein by reference, discloses a proposed method for breaking a fluid utilizing a percarbonate, perchlorate or persulfate breaker encapsulated with a polyamide. The polyamide membrane is permeable to at least one fluid in the formation which dissolves the breaker and the breaker then diffuses through the membrane to break the fracturing fluid with the membrane staying intact during the breaker release. Thus providing a means of slowly releasing amounts of breaker over time instead of a single release of the total volume of the breaker from all capsules at a given time.
U.S. Pat. No. 5,373,901, incorporated herein by reference, discloses a method of encapsulating a breaker within a membrane comprising a partially hydrolyzed acrylic crosslinked with either an aziridine prepolymer or a carbodiimide. The membrane has imperfections through which the breaker can diffuse upon contact with an aqueous fluid. The imperfections may be created by the incorporation of selected micron-sized particles in the membrane coating.
U.S. Pat. No. 5,437,331, incorporated herein by reference, discloses a polymeric particle or bead having a network of pores with an enzyme breaker held protectively within the network to provide a controlled time release of the enzyme. The invention is described as having increased mechanical stability over previous micro-encapsulated or gel delivery vehicles, which renders this delivery system capable of being manufactured, processed, handled, and applied under more severe conditions, such as mechanical pumping.
U.S. Pat. No. 5,580,844, incorporated herein by reference, provides a coated breaker chemical, in which the coating comprises a blend of neutralized sulfonated ionomer and asphalt. Such coatings were shown to be useful because of their water barrier properties, their elasticity, and ability to be applied as thin continuous coatings substantially free of pinholes. The patent describes the capability of this encapsulation to include enzyme breakers, and to provide controlled release of the breaker over a period of time under conditions of use.
U.S. Pat. No. 5,591,700, incorporated herein by reference, discloses a breaker encapsulated by a water soluble surfactant. The surfactants proposed are waxy materials that melt and/or dissolve into the fracturing fluids at temperatures in the subterranean formation to be fractured. The distinguishing feature of these surfactants is that they are solid at ambient surface conditions, while dissolving at temperatures within the formation.
Further, U.S. Pat. No. 5,604,186, incorporated herein by reference, describes an enzyme solution coated substrate covered with a membrane comprising a partially hydrolyzed acrylic crosslinked with either an azidirine prepolymer or carbodiimide. The membrane contains imperfections through which an aqueous fluid may pass into the breaker to contact the enzyme and diffuse the enzyme outward from the breaker particle.
U.S. Pat. No. 5,948,735, incorporated herein by reference, discloses an encapsulated breaker for use in oil-based fracturing fluids. The invention describes a solid particle breaker chemical coated with an oil degradable rubber coating, which is introduced into an oil-based fracturing fluid, which exhibits a delayed release of the active chemical.
As described in the previously-mentioned patents, certain types of encapsulation can be useful to inactivate a breaker until such time, or under such conditions, as the chemical activity is needed to decrease viscosity of the fracturing fluid. As described in U.S. Pat. No. 5,806,597, encapsulation has its limitations. For instance, premature release of the enzyme payload sometimes occurs due to product manufacturing defects, imperfections, or coating damage experienced in pumping the particles through surface equipment tubular and perforations.
U.S. Pat. No. 5,806,597 (xe2x80x9cthe ""597 patentxe2x80x9d), incorporated herein by reference, proposes that rather than encapsulate the breaker, a complex containing the breaker is maintained in a substantially unreactive state by maintaining conditions of pH and temperature. The complex comprises a matrix of compounds, substantially all of which include a breaker component, a crosslinker component, and a polymer component. Once the fracture is completed, conditions are changed, the complex becomes active, and the breaker begins to catalyze polymer degradation.
Further, the ""597 patent discloses that the preferred breaker components are polymer specific enzymes. These enzymes are particularly advantageous in that they will attach to a strand of the polymer, although inactive, and bind or stay attached to that polymer until such time as conditions are appropriate for the reaction to occur. The enzyme will migrate with the substrate, such that it will be dispersed within the fluid where it is needed.
The underlying basis of this method of control is better explained by considering conventional enzyme pathways which may be described by the following reaction: E+S∴[ES]∴E+P, in which E is an enzyme, S is a substrate, [ES] is an intermediate enzyme-substrate complex and P is the product of the substrate degradation catalyzed by the enzyme. The reaction rate of the intermediate enzyme-substrate complex is pH dependent and may be slowed or even virtually halted by controlling the pH and temperature of the enzyme substrate complex. Further explanation of this process may be found in MALCOM DIXON and EDWIN C. WEBB, ENZYMES 162 (1979).
Although the literature reflects a great deal of effort directed at controlling the activity of fracturing fluid breakers, most of those methods are limited in their usefulness by unfavorable downhole conditions or by economic factors. Particularly lacking in the field are adequate ways of avoiding the problems associated with drilling fluids, which must undergo high shear while drilling, cycling of temperature between bottom-hole and surface, and remain useable for weeks. Once drilling stops, the residues, or filter cakes remaining in the well, that inhibit drilling operations or damage producing formations, must be destroyed, sometimes at an indeterminate time after drilling. Still needed are better ways of providing a functional agent, such as an enzyme or a chemical, that can withstand the rigors of drilling, be deliverable to a specified downhole location and of obtaining a desired or selective activity to accomplish the decomposition of a polymeric viscosifier, or other substrate. Also needed are better ways of controlling the release or activity of an enzyme, chemical or other functional agent in order to alter the physical or chemical properties of a polymeric component of an oil field fluid or residue. Moreover, suitable physically robust particles that respond to a trigger to release an enzyme or otherwise reactive substance that has been held inactive would have a number of applications. Such particles could also lend themselves to solving the more general problems of building in countermeasures to fluid contamination, selectable degradation of solid materials within and without the well bore, and facilitation of waste management of materials containing degradable polymers.
The present invention solves many of the problems encountered in the hydrocarbon exploitation industry. The inventors have developed active, and particularly catalytic, agents that can be made inert and remain inert under shear, temperature and prolonged exposure and that can be safely added to materials which would otherwise quickly change physical or chemical properties in their presence. Yet those inert agents become active to make those changes in response to a stimulus or trigger delivered either by direct action or the action of environmental agents made accessible over time or as a result of some indirect change such as reversal of pressure differentials or discharge into the environment. The agent, such as an enzyme or radical initiator, once activated is able to reverse physical or chemical properties (e.g., breaking the seal of an impermeable filter cake to release gas and oil or converting a mechanically strong material into innocuous fragments) has wide applications to the problems of building in countermeasures to fluid contamination, selectable degradation of solid materials within and without the well bore, and facilitation of waste management of materials containing degradable materials.
Accordingly, certain embodiments of the invention are directed to methods and related compositions for altering the physical and/or chemical properties of substrates used in hydrocarbon exploitation, in both downhole and in surface applications. These compositions and methods will find use in a variety of drilling, completion, workover, production, reclamation and disposal operations. The more preferred embodiments include the triggered release of agents, such as enzymes and chemicals that specifically act on defined substrates, such as polymeric viscosifiers, fluid loss control agents and chemical contaminants like H2S. Creating a new drilling fluid formulation, including an enzyme within the circulating fluid system could provide for easy decomposition of the drilling fluid at the end of drilling operations, both in the fluid returned to tanks on the surface and the fluid lost to the formation or discharged whole or on cuttings into the environment. In certain of the new reservoir drilling fluid compositions, the encapsulated enzyme retains the enzyme during drilling operations and releases the enzyme or enzymes upon receipt of a chemical trigger such as pH or salinity change, or the enzyme is released over a defined period of time. An important trigger has been found to be CO2, which is present in many reservoirs.
In accordance with certain embodiments of the present invention, a method of degrading a predetermined substrate is provided. The method includes formulating a fluid or a solid material containing a degradable substrate and an inactivated substrate-degrading agent, the inactivated agent being responsive to a predetermined triggering signal such that the agent becomes activated upon exposure to the triggering signal. The activated agent is capable of degrading the substrate under degradation promoting conditions to change its physical or chemical properties. In some embodiments the step of applying a triggering signal comprises exposing the inactivated degrading agent to a stimulus selected from the group consisting of exposure to a reducing agents, oxidizers, chelating agents, radical initiators, carbonic acid, ozone, chlorine, bromine, peroxide, electric current, ultrasound, change in pH, change in salinity, change in ion concentration, change in temperature and change in pressure, the inactivated degrading agent being capable of physically and/or chemically responding to said stimulus.
In some embodiments the degrading agent comprises at least one enzyme having activity for degrading the substrate under degradation promoting conditions, and in some embodiments the substrate-degrading agent is encapsulated by an encapsulating material that is responsive to said triggering signal such that at least a portion of said enzyme is released by said encapsulating material upon exposure to a triggering signal. Certain embodiments include an encapsulating material formed of a co-polymer of (a) an ethylenically unsaturated hydrophobic monomer with (b) a free base monomer of the formula
CH2xe2x95x90CR1COXR2NR3R4
where R is hydrogen or methyl, R2 is alkylene containing at least two carbon atoms, X is O or NH, R3 is a hydrocarbon group containing at least 4 carbon atoms and R4 is hydrogen or a hydrocarbon group. In certain embodiments R3 is t-butyl and R4 is hydrogen, and in certain embodiments R1 is methyl, R2 is ethylene and X is O. In some embodiments the hydrophobic monomer is a styrene or methylmethacrylate, and the encapsulating material is a co-polymer of styrene or methyl methacrylate with t-butyl amino ethyl methacrylate. In some embodiments the co-polymer comprises 55 to 80 weight % styrene, methyl styrene or methyl methacrylate with 20 to 45 weight % t-butylamino-ethyl methacrylate.
According to certain embodiments, the method also includes maintaining enzyme activity promoting conditions in a downhole environment, and, optionally, establishing enzymatic activity inhibiting conditions. In some embodiments the fluid or solid device comprises at least two inactivated enzymes, wherein the inactivated enzymes are capable of being reactivated by the same or different triggering signals, such that upon reactivation the reactivated enzymes are capable of acting upon the same or different substrates independently or in concert. In some embodiments the enzyme is selected from the group consisting of endo-amylases, exo-amylases, isomylases, glucosidases, amylo-glucosidases, malto-hydrolases, maltosidases, isomalto-hydro-lases and malto-hexaosidases. In some embodiments the reactivated enzyme is capable of being inactivated by application of a second triggering signal, wherein the second triggering signal may be the same or a different triggering signal, such that the inactivated enzyme no longer acts on the substrate.
Certain embodiments of the methods of the invention employ a degradable substrate selected from the group consisting of celluloses, derivatized celluloses, starches, derivatized starches, xanthans and derivatized xanthans. In certain embodiments the fluid is a circulating drilling fluid, completion fluid or workover fluid. In some embodiments the fluid is a stimulation fluid such as a fracturing fluid. In other embodiments the may include formulating a solid device comprises a self-destructing bridging particle containing a degradable substrate and a reactivatable inactivated enzyme for reversible fluid loss control. In some embodiments the method employs a solid device comprises degradable polymers and a reactivatable inactivated enzyme fashioned into hardware for use downhole or on the surface.
According to another embodiment, a method of increasing the flow of production fluid from a well is provided that comprises formulating a fluid comprising a degradable polymeric substrate and an inactivated enzyme. This method also includes introducing the fluid into a downhole environment and applying a triggering signal to the fluid. The triggering signal is sufficient to reactivate the inactivated enzyme to give a reactivated enzyme, and the reactivated enzyme is capable of selectively degrading the substrate sufficient to alter a physical property of the fluid such that the flow of production fluid is increased. In some embodiments the step of introducing the fluid into a downhole environment comprises forming a filter cake containing said degradable substrate and said inactivated enzyme. In some embodiments the fluid comprises more than one inactivated enzyme, wherein the inactivated enzymes are capable of being reactivated by the same or different triggering signals, wherein upon reactivation the reactivated enzymes are capable of acting upon the same or different substrates. In some embodiments the fluid is a circulating drilling fluid, a completion fluid, a workover fluid or a stimulation fluid. According to another embodiment, a method of increasing the flow of production fluid from a well is provided that comprises formulating a fluid comprising a degradable polymeric substrate and an inactivated enzyme. This method also includes introducing the fluid into a downhole environment, where the fluid is present as whole fluid, such as drilling fluid lost to natural fractures and other open features. The direct application of a physical, triggering signal, such as a change in pH with weak acids, is sufficient to reactivate the inactivated agent, such as an enzyme, to give a reactivated enzyme, and the reactivated enzyme is capable of selectively degrading the substrate sufficient to alter a physical property of the fluid as viscosity or particle suspending ability or pore-plugging ability such that the flow of production fluid is increased. Cementing and other activities that indirectly increase fluid production can also benefit by, for example, liquefaction and sloughing of drilling fluids left behind by imperfect cleaning of the well bore.
Carbon dioxide, present in many producing formations, has been shown to be an effective trigger for certain formulations. This provides for indirect delivery of the trigger by the reversal of pressure at the time of production. During drilling, completion, stimulation, and workover operations, the pressure is usually in the radially out direction, forcing fluids out from the wellbore and pushing formation fluids away form the borehole. Production begins with a reversal of the pressure differential, inducing formation fluids to flow into the well bore. Fluids inadvertently or purposefully left in the well bore become more exposed to the formation fluids, very often including CO2. In contact with an aqueous phase, CO2 reacts with water to form carbonic acid H2CO3, a mild acid, but sufficient to lower the pH of fluids to the bicarbonate buffer point determined by the environment.
Also provided by the present invention is a method of degrading filter cake. The method comprises formulating a fluid capable of making filter cakes and comprising a polymeric viscosifier or fluid loss control agent and an inactivated enzyme. An important example is a drilling fluid, where filter cake formation is an essential feature. The fluid is introduced into a downhole environment such that a filter cake containing the polymeric viscosifier or fluid loss control agent and the inactivated enzyme is formed. The fluid may be displaced from the well at that point, leaving the solid filter cake pressed into the surface of the well bore. A triggering signal is applied to the filter cake, the triggering signal being sufficient to reactivate the inactivated enzyme to give a reactivated enzyme. The reactivated enzyme is capable of selectively degrading the polymeric viscosifier or fluid loss control agent such that the filter cake at least partially disintegrates, allowing fluid to pass through the previously impermeable cake. CO2 from the formation provides an especially useful route for decomposition of filler cakes where externally applied breakers such as concentrated mineral acids or oxidizers cannot be used, or where no external wash can be applied due to, for example, mechanical failure, preventing even application of the intended trigger signal.
Further provided by the present invention is a method of eliminating a contaminant from a drilling fluid or subterranean formation. According to certain embodiments, a fluid is formulated that comprises an inactivated contaminant-destroying agent. The method includes introducing the fluid into a downhole environment containing a predetermined contaminant that is a substrate capable of being degraded or destroyed by the agent under degradation promoting conditions, and then applying a triggering signal to the fluid. The optimal signal is the appearance of the contaminant, such as the lowering of pH by the introduction of hydrogen sulfide. The triggering signal then reactivates the inactivated agent to allow it to degrade the contaminant. As it often takes more than an hour for fluids to circulate from the bottom of a well to the top, and fluids are often left standing statically in the well, such a contaminant-triggered response provides for an automatic response, using materials that would otherwise be consumed by side reactions or destroy other fluid components if active in the fluid. The method may also include dislodging a piece of drilling equipment from an at least partially disintegrated filter cake.
Further provided by the present invention is a method of eliminating a contaminant from a drilling fluid or subterranean formation. According to certain embodiments, a fluid is formulated that comprises an inactivated substrate-degrading agent. The method includes introducing the fluid into a downhole environment containing a predetermined contaminant that is a substrate capable of being degraded by the agent under degradation promoting conditions, and then applying a triggering signal to the fluid. The triggering signal is sufficient to reactivate the inactivated agent to provide a reactivated agent. allowing the reactivated substrate-degrading agent to degrade the contaminant. The fluid may be, for example, a circulating drilling fluid, completion fluid or a workover fluid and, in certain embodiments the contaminant is H2S.
Also provided in accordance with the present invention is a wellbore servicing composition comprising a fluid or a solid device containing at least one degradable substrate, said substrate contributing to the structural integrity of said device or to the structural integrity of a residue of said fluid, and an inactivated substrate-degrading agent. The substrate-degrading agent is capable of responding to a triggering signal such that the agent becomes at least partially reactivated sufficient to degrade said substrate under degradation promoting conditions in a downhole environment such that a physical or chemical property of the composition is altered. The utility of the invention in destroying solid filter cake formed in the wellbore and containing the inactivated agent can be extended to pre-formed solid materials. An example would be to make solid particles from starch and starch-containing synthetic polymers to serve a rigid bridging particles, for example, for use in low density fluids where the density of calcium carbonate cannot be tolerated, and strong chemicals cannot be used to clean up the filter cake, or where cleanup chemicals may not be able to be applied. Another application could be to cash sheets of degradable polymer containing the inactivated agent for use as cover for premium screens such as prepacked sand screens. The covers could prevent damage of the screens whilst being placed into the wellbore, and then destroyed by application of the trigger or exposure to CO2 from the well.
Still further provided in accordance with the invention is a wellbore treatment method comprising formulating a fluid comprising an encapsulated substrate-degrading agent; introducing the fluid into a downhole environment containing a predetermined substrate capable of being degraded by the agent under degradation promoting conditions; and providing for generation of the trigger upon reaching the desire point. One example would be the use of encapsulation to preserve the activity of the agent that would normally be lost during the trip to the site of use, say by thermal degradation of enzymes in a brine pumped to the producing zone at the bottom of a deep, hot well. Including materials that generate a trigger as they thermally degrade would provide for the preserved agent to be released where it could immediately act.
Also provided by the present invention is a composition for use in hydrocarbon exploitation operations. The composition can be, for example, a circulating drilling fluid, a completion fluid, a workover fluid, a bridging particle and a solid hardware device. In certain embodiments the composition comprises a fluid or a solid device containing at least one degradable substrate and an encapsulated substrate-degrading agent. The encapsulated agent is capable of responding to a triggering signal such that the agent becomes sufficiently unencapsulated to allow the agent to degrade the substrate under degradation promoting conditions such that a physical or chemical property of the substrate is altered. In some embodiments the encapsulated substrate-degrading agent is inactivated by encapsulation in a material that is capable of responding to the triggering signal by making the degrading agent available to the degradable substrate. In certain embodiments the triggering signal includes a change in pH of a medium contacting the encapsulated agent. The substrate degrading agent may comprise at least one inactivated enzyme, wherein the inactivated enzymes are capable of being reactivated by the same or different triggering signals, wherein upon reactivation the reactivated enzymes are capable of acting independently or in concert upon the same or different substrates. In some embodiments the substrate is selected from the group consisting of celluloses, derivatized celluloses, starches, derivatized starches, xanthans, and derivatized xanthans. In some embodiments the substrate contributes to the structural integrity of the device or to the structural integrity of a residue of the fluid such that degradation of a substrate causes a physical change in the composition. For instance, the disintegration of a filter cake. In some embodiments the enzyme is an endo-amylase, exo-amylase, isomylase, glucosidase, amylo-glucosidase, malto-hydrolase, maltosidase, isomalto-hydro-lase or malto-hexaosidase.
In certain embodiments, the triggering signal comprises exposure to a reducing agent, oxidizer, chelating agent, radical initiator, carbonic acid, ozone, chlorine, bromine, peroxide, electric current, ultrasound, change in pH, change in salinity, change in ion concentration, change in temperature and change in pressure, or a combination of such stimuli.
In some composition embodiments the encapsulated agent comprises an encapsulation material formed of a co-polymer of (a) an ethylenically unsaturated hydrophobic monomer with (b) a free base monomer of the formula
CH2xe2x95x90CR1COXR2NR3R4
where R is hydrogen or methyl, R2 is alkylene containing at least two carbon atoms, X is O or NH, R3 is a hydrocarbon group containing at least 4 carbon atoms and R4 is hydrogen or a hydrocarbon group. For example, the encapsulating material may be a co-polymer of styrene or methyl methacrylate with t-butyl amino ethyl methacrylate.
These and other features of the present invention are more fully set forth in the description of illustrative embodiments of the invention with reference to the following drawings.